1. Field of the Invention
The present invention generally relates to an anti-lock brake control system for preventing lockup of vehicle wheel brakes during braking of the vehicle, and more particularly, to an anti-lock modulator constituting part of the anti-lock brake control system of a recirculation type, which is adapted to prevent pulsations of delivery pressure from a pump to a master cylinder. In the anti-lock brake control system, working fluid is discharged to a recirculating passage from wheel brakes when the anti-lock brake control is effected, so that the working fluid is recirculated to the master cylinder by the pump provided in the recirculating passage.
2. Description of the Prior Art
Nowadays, the use of an anti-lock brake control system has been spreading in general automobiles except relatively small ones. Accordingly it is urgently required to make a system which is also applicable to small automobiles, with reduced size and manufacturing cost.
Japanese Patent Publication No. 49-28307 (German Patent No. 719446) discloses such an anti-lock brake control system as referred to above for satisfying the recent requirement. The system disclosed therein includes;
(a) a recirculating passage branched off from a main fluid passage connecting a master cylinder and wheel brakes so as to recirculate the working fluid back to a recirculation point located upstream of the branch point in the main fluid passage with respect to the direction of flow towards the wheel brakes, PA1 (b) a pump provided in the recirculating passage for recirculating the working fluid, and PA1 (c) a pressure adjusting means which reduces the pressure applied to the wheel brakes by discharging the working fluid from the branch point to the recirculating passage, and increases the pressure to the wheel brakes by supplying the working fluid from the main fluid passage to the wheel brakes through the branch point.
The system of this kind will be referred to as a recirculating type hereinbelow.
In the anti-lock brake control system mentioned above, the pressure adjusting means is provided with a solenoid-operated input valve in the main fluid passage between the recirculation point and the branch point and a solenoid-operated output valve in the recirculating passage between the branch point and the pump. The input and out valves are normally open and closed, respectively. Therefore, the brake pressure is controlled when the anti-lock brake control is being performed by switching these two valves in three modes, that is, pressure reducing mode, pressure holding mode and pressure increasing mode.
Similar to the above anti-lock brake control system of the recirculating type, British Patent Application No. 8512610 proposes a system for performing the anti-lock brake control in two modes, namely, a pressure reducing mode and a pressure gradually increasing mode using one solenoid valve for each one wheel. In this system, a flow control valve which is not solenoid-operated is used in place of the solenoid-operated input valve.
According to the conventional anti-lock brake control systems of the recirculating type described above, when the brake pressure is reduced in the anti-lock operation, the working fluid discharged out of each wheel brake is recirculated between the master cylinder and the pressure adjusting means by the pump. Because of this, there arise several problems, for example, vibrations of a brake pedal resulting from pulsations of delivery pressure from the pump, noises caused by vibrations of pipings, or the like.
In view of the above, various improvements have been proposed to reduce the transmission of pressure pulsations from the pump towards the master cylinder. For example, a throttle is provided between an outlet of the pump and the recirculation point, with an accumulator arranged between the throttle and the outlet of the pump as disclosed in German Patent Application No. 2643860. Japanese Patent Publication Nos. 56-142733 and 61-16656 disclose such an improved method where a check valve is provided between the recirculation point and the master cylinder so as to normally direct the working fluid from the master cylinder to the recirculation point, thereby preventing the transmission of the delivery pressure of the pump to the master cylinder. On the other hand, Japanese Utility Model Laid-open Publication No. 63-98869 proposes to prevent the transmission of the pressure pulsations from the pump to the master cylinder by utilizing a piston operated by the delivery pressure of the pump. When the pump is driven, the piston is moved to restrict the fluid passage through the master cylinder and the recirculation point with a throttle.
However, the above-described conventional methods for preventing the transmission of the pressure pulsations of the pump have encountered various problems.
More specifically, according to the method disclosed in German Patent Application No. 2643860, the total amount of the working fluid discharged from the pump must pass through the throttle, requiring that the delivery pressure of the pump be increased, thereby imposing a larger load upon the pump. Further, the housing structure is larger in size because of the high pressure accumulator. Additionally, a plug and the like are necessitated.
According to the method of Japanese Patent Publication No. 56-142733, an accumulator is also required between the outlet of the pump and the check valve, resulting in a bulky system structure and increased manufacturing cost.
Moreover, according to the method disclosed in Japanese Utility Model Laid-open Publication No. 63-98869, when the normal braking operation is carried out, the piston is moved, resulting disadvantageously in an increased stroke of the brake pedal. In addition, the structure becomes complicated and the manufacturing cost increases.